Leicester Crown Court was told the defendant was suffering from PTSD and "lost control" when he attacked his victim at his home in Coalville.
The defence said the PTSD was triggered by abuse Mr McCullum suffered at the hands of older children when he was younger.
Giving evidence in his own defence, the 30-year-old defendant said was raped in woodland by an older boy when he was eight or nine.
Mr McCullum told the court he had gone with a group of older children to Whitwick Woods in Leicestershire where he was attacked.
He said: "I thought they were my friends. They were nice to me. They would buy me sweets.
Ross McCullum said he had been attacked as a child by Stephen Beadman, who would later go on to rape and murder 15-year-old Kayleigh Haywood
Mr McCullum also told the jury there were two other occasions where he was made to perform sexual acts in the garage of his home, in Windsor Close, Coalville, by older children who lived nearby.
He was jailed for life with a minimum term of 35 years in 2016 and died in prison in April 2021, aged 28.
Mr McCullum told the court Beadman was there when he had been made to perform sexual acts in the garage.
"I was trying to get my trousers back up but Stephen was whipping me with it saying 'you better not tell anyone'," said Mr McCullum.
The court has heard that Mr McCullum and Ms Newborough, from Nuneaton, had been in a short relationship before he killed her.
They had met at Ibstock Brick, in Ibstock Leicestershire, where she worked in human resources and he was a laboratory technician.
They had arranged to meet at Mr McCullum's home when his parents were out and he strangled her, cut her throat, and dumped her body in undergrowth by a country lane near Woodhouse Eaves.
The prosecution said Mr McCullum continued to text and call Ms Newborough's phone feigning concern for to cover his tracks and hid what he had done.